This seems purely cosmetic, but annoying nonetheless. Occasionally when viewing a job page, one or more of the currently building jobs will disappear from the build history sidebar. A refresh of the page will usually bring them back, though not for very long.
I think this differs from all the other "builds disappearing from build history" tickets in that a browser refresh shows them again (at least temporarily) and it's only when the build is occurring (blinking ball with progress bar); no completed builds have been observed as disappearing.
This has been observed on the latest versions of Chrome on Windows and Mac, and Safari on Mac.